Horizontal lines showing in volume scatter shader
Volumetrics shader are simulated by "overlaying several layers of thickness". These are called samples. From the Blender reference manual's page: 


  Step Size: Distance between volume shader samples when rendering the volume. Lower values give more accurate and detailed results but also increased render time.


Here's a default UV sphere showing the steps:



Your scene is quite big and deep, so you'll have to decrease the distance (increase the resolution) between these "planes" with the Step size property you can find in the Volume sampling panel in Render tab to hide this effect.

The purpose is to get a finer calculation of the procedural texture you are plugging in.